Cancer cells must acquire multiple mutations to evade the many restraints that prevent uncontrolled proliferation. Normal cells are vigilant in maintaining genome integrity (i.e. preventing mutations), but one hallmark of cancer cells is a defect in such self- surveillance and hence genome instability. Checkpoint controls that govern entry into mitosis are essential for maintaining genome integrity during cell proliferation. The Wee1-family kinases that inhibit Cdc2 are among the targets for these checkpoints both in mammalian cells and in yeast, but the basis for their regulation remains unclear. In budding yeast, the Wee1-relative Swe1p is employed by the morphogenesis checkpoint, responding to perturbations of cell shape and the actin cytoskeleton. We have identified a novel pathway controlling Swe1p degradation, whose initiation depends on a family of cytoskeletal proteins called septins. Septin organization appears to respond to cell shape, and can regulate a checkpoint kinase called Hsl1p. One aim of this proposal is to understand how septins are organized and how that organization responds to cell geometry to act as a shape sensor. In addition to cell shape, the checkpoint responds to several physiological stresses, and a second aim of the proposal is to understand how these stresses regulate swe1p degradation and hence cell cycle progression. Although a morphogenesis checkpoint has not yet been identified in cells other than yeast, appropriate responses of mammalian cells to geometrical and mechanical features of their microenvironment are thought to involve sensing of similar types of cytoskeletal information. The metastatic potential of cancer cells increases upon derangement of these pathways, and the proposed studies will therefore provide new insight into cancer progression, and may yield potential targets for therapeutic intervention.